For LCD devices, a classification based on the operating mode of liquid crystal molecules includes a phase change (PC) mode, a twisted nematic (TN) mode, a super twisted nematic (STN) mode, an electrically controlled birefringence (ECB) mode, an optically compensated bend (OCB) mode, an in-plane switching (IPS) mode, a fringe field switching (FFS) mode, a vertical alignment (VA) mode and a polymer sustained alignment (PSA) mode. A classification based on the driving mode of the device includes a passive matrix (PM) type and an active matrix (AM) type. The PM type is classified into static type, multiplex type and so on, and AM type is classified into a thin film transistor (TFT) type, metal insulator metal (MIM) type and so on. The TFT type is further classified into amorphous silicon type and polysilicon type, wherein the latter is classified into a high-temperature type and a low-temperature type according to the production process. A classification based on the light source includes a reflective type utilizing natural light, a transmissive type utilizing a backlight and a transflective type utilizing both the natural light and a backlight.
The devices include a liquid crystal composition having suitable characteristics. The liquid crystal composition has a nematic phase. General characteristics of the composition should be improved in order to obtain an AM device having good general characteristics. Table 1 below summarizes the relationship between the general characteristics of the two aspects. The general characteristics of the composition will be further described based on a commercially available AM device. The temperature range of the nematic phase relates to the temperature range in which the device can be used. A preferred maximum temperature of a nematic phase is about 70° C. or higher and a preferred minimum temperature of a nematic phase is about −10° C. or lower. The viscosity of a composition relates to the response time of the device. A short response time is preferred for displaying moving images on the device. Hence, a small viscosity in the composition is preferred. A small viscosity at a low temperature is further preferred.
TABLE 1General Characteristics of Composition and AM DeviceGeneral CharacteristicsGeneral CharacteristicsNo.of Compositionof AM Device1Wide temperature range of aWide usable temperature rangenematic phase2Small viscosityShort response time3Suitable optical anisotropyLarge contrast ratio4Large positive or negativeLow threshold voltage anddielectric anisotropysmall electric power consumption,Large contrast ratio5Large specific resistanceLarge voltage holding ratio andlarge contrast ratio6High stability to UV light andLong service lifeheat
The optical anisotropy (Δn) of the composition relates to the contrast ratio of the device. The product (Δn×d) of the Δn of the composition and the cell gap (d) of the device is designed so as to maximize the contrast ratio. A suitable value of the product depends on the type of the operating mode. In a device of the VA or PSA mode, the suitable value is in the range of about 0.30 μm to about 0.40 μm, and in a device of the IPS mode, the suitable value is in the range of about 0.20 μm to about 0.30 μm. In the above case, a composition having a large Δn is preferred for a device having a small cell gap. A large absolute value of the dielectric anisotropy (Δ∈) of the composition contributes to a low threshold voltage, a small electric power consumption and a large contrast ratio of the device. Accordingly, a large absolute value of Δ∈ is preferred. A large specific resistance of the composition contributes to a large voltage holding ratio, and to a large contrast ratio of the device. Accordingly, a composition having a large specific resistance at room temperature and also at a high temperature in an initial stage is preferred. A composition having a large specific resistance at room temperature and also at a high temperature even after the device has been used for a long period of time is preferred. Stability of the composition to UV light and heat relates to the service life of the LCD device. In a case where the stability is high, the device has a long service life. Such characteristics are preferred for an AM device for use in a liquid crystal projector, a liquid crystal television and so forth.
A composition having a positive Δ∈ is used for an AM device of the TN mode. On the other hand, a composition having a negative Δ∈ is used for an AM device of the VA mode. A composition having a positive or negative Δ∈ is used for an AM device of the IPS or FFS mode. A composition having a positive or negative Δ∈ is used for an AM device of the PSA mode. Examples of the liquid crystal composition having a negative Δ∈ are disclosed in Patent literature Nos. 1 to 6 described below and so on for an AM device of PSA mode.
A desirable AM device has characteristics such as a wide temperature range in which the device can be used, a short response time, a large contrast ratio, a low threshold voltage, a large voltage holding ratio and a long service life. A shorter response time even by one millisecond is desirable. Thus, desirable characteristics of a composition include a high maximum temperature of nematic phase, a low minimum temperature of nematic phase, a small viscosity, a suitable optical anisotropy, a large positive or negative dielectric anisotropy, a large specific resistance, a high stability to UV light, and a high stability to heat.
A liquid crystal composition containing a polymer is used in an LCD device of the PSA mode. First, a liquid crystal composition to which a small amount of polymerizable compound is added is injected into the device. Next, the composition is irradiated with UV light, while a voltage is applied between substrates of the device, to polymerize the polymerizable compound and form a polymer network in the composition. In the composition, alignment of liquid crystal molecules can be controlled by the polymer, and therefore the response time of the device is shortened and image persistence is reduced. Such effect of the polymer can also be expected for a device of a mode such as the TN, ECB, OCB, IPS, FFS or VA mode.
In a general PSA-mode LCD device, a polyimide alignment film is formed on surfaces of a TFT substrate and a counter substrate, and then a liquid crystal is sealed into the space between the TFT substrate and the counter substrate. Meanwhile, Patent literature Nos. 7 to 9 and Non-patent literature No. 1 report a technology in which a step for forming an alignment film can be omitted. In the technology, a liquid crystal in which a polymerizable compound such as a (meth)acrylate is added is sealed into a space between a TFT substrate and a counter substrate, and then the polymerizable compound is polymerized by exposure to UV light to form on a substrate surface an alignment control layer for aligning liquid crystal molecules in a vertical or horizontal direction. An LCD in which the alignment control layer is formed by the technology is referred to as an alignment film-less mode LCD device.
In the alignment film-less mode LCD device described above, alignment regulation force in the alignment control layer is required to be satisfactory. Insufficiency of the alignment regulation force in the alignment control layer causes a decrease of the contrast ratio and light leakage. Non-patent literature No. 1 reports that a high-brightness LCD device can be constituted by a technique using two kinds of polymerizable compounds. However, the magnitude of the contrast ratio of the display device is far from sufficient.
Moreover, the alignment film-less mode LCD device has a merit of allowing omission of a step for forming an alignment film. However, the polyimide alignment film for accelerating the rate of the polymerization of (meth)acrylate is not formed in the mode, so the mode has a problem of insufficient polymerization reactivity. When the polymerization reactivity is insufficient, the “unreacted polymerizable compound,” which is known to cause image persistence and so on, increases after a step for forming a pretilt.
Patent literature No. 9 studies a technique using a polymerizable compound and a self-alignment additive such as a steroid-based alcohol or a terphenyl-based amine as a material for an alignment control layer, succeeding in obtaining good alignment properties. However, the technique has a problem of low compatibility of the self-alignment additive with any other liquid crystal composition, which causes precipitation and so on during transportation or production of the LCD device to cause a content change or a poor display.
Accordingly, in the alignment film-less mode LCD device, the alignment control material is desirably excellent in the compatibility with any other component, and the polymerization reactivity of the polymerizable compound is desirably satisfactory.
Patent literature No. 1: JP 2003-307720 A.
Patent literature No. 2: JP 2004-131704 A.
Patent literature No. 3: JP 2006-133619 A.
Patent literature No. 4: EP 1889894 A.
Patent literature No. 5: JP 2010-537010 A.
Patent literature No. 6: JP 2010-537256 A.
Patent literature No. 7: JP 4515102 B.
Patent literature No. 8: JP 2005-208309 A.
Patent literature No. 9: WO 2012-038026 A.
Non-patent literature No. 1: Li et al., International Display Workshop (IDW) '09 vol. 1 p. 23 (2009)